Luana Fioriti Research Associate Scholar The Italian Academy for Advanced Studies in America at Columbia University Weekly Seminar of the Fellows Program April 11th, 2007 Cytoplasmic polyadenylation element binding protein (CPEB): a prion-like protein as a regulator of local protein synthesis and synaptic plasticity 1.INTRODUCTION With this paper I would like to describe you what is my research project here at Columbia and how I am trying to address the many questions underlying my project by working everyday in the lab. But before doing this I feel somehow obliged to give you an introduction on the basic concepts of neurobiology. Therefore we will start with a brief definition and description of what is a neuron, how neurons interact to form synapse and neural circuits, how synapse activity can be modified and finally how these changes in synaptic activity underlie high cognitive processes such as learning and memory. After providing you this, I hope not too boring introduction, I will go deeper into the molecular aspects of these phenomenon and I will illustrate you the main goal of my research, which is to characterize the role of a particular protein called Cytoplasmic Polyadenylation Element Binding protein with respect to the morphological and physiological changes that occur at the synapse after neuronal stimulation. Memory In psychology, memory is an organism's ability to store, retain, and subsequently recall information. Although traditional studies of memory began in the realms of philosophy, the late nineteenth and early twentieth century put memory within the paradigms of cognitive psychology. In recent decades, it has become one of the principal pillars of a new branch of science called cognitive neuroscience, a marriage between cognitive psychology and neuroscience. There are several ways to classify memories, based on duration, nature and retrieval of information. From an information processing perspective there are three main stages in the formation and retrieval of memory: • Encoding or registration (processing and combining of received information) • Storage (creation of a permanent record of the encoded information) • Retrieval or recall (calling back the stored information in response to some cue for use in a process or activity) Classification A basic and generally accepted classification of memory is based on the duration of memory retention, and identifies three distinct types of memory: sensory memory, short term memory and long term memory. Sensory Sensory memory corresponds approximately to the initial 200 - 500 ms after an item is perceived. The ability to look at an item, and remember what it looked like with just a second of observation, or memorization, is an example of sensory memory. With very short presentations, participants often report that they seem to "see" more than they can actually report. The first experiments exploring this form of sensory memory were conducted by George Sperling using the "partial report paradigm." Subjects were presented with a grid of 12 letters, arranged into three rows of 4. After a brief presentation, subjects were then played either a high, medium or low tone, cuing them which of the rows to report. Based on these partial report experiments, Sperling was able to show that the capacity of sensory memory was approximately 12 items, but that it degraded very quickly (within a few hundred milliseconds). Because this form of memory degrades so quickly, participants would see the display, but be unable to report all of the items (12 in the "whole report" procedure) before they decayed. This type of memory cannot be prolonged via rehearsal. Short-term Some of the information in sensory memory is then transferred to short-term memory. Short-term memory allows one to recall something from several seconds to as long as a minute without rehearsal. Its capacity is also very limited: George A. Miller, when working at Bell Laboratories, conducted experiments showing that the store of short term memory was 7±2 items (the title of his famous paper, "The magic number 7±2"). Modern estimates of the capacity of short-term memory are lower, typically on the order of 4-5 items, and we know that memory capacity can be increased through a process called chunking. For example, if presented with the string: FB IPH DTW AIB M people are able to remember only a few items. However, if the same information is presented in the following way: FBI PHD TWA IBM people can remember a great deal more letters. This is because they are able to chunk the information into meaningful groups of letters. Beyond finding meaning in the acronyms above, Herbert Simon showed that the ideal size for chunking letters and numbers, meaningful or not, was three. This is evidenced by the tendency to remember phone numbers as several chunks of three numbers with the final four-number groups generally broken down into two groups of two. Short-term memory is believed to rely mostly on an acoustic code for storing information, and to a lesser extent a visual code. Conrad (1964) found that test subjects had more difficulty recalling collections of words that were acoustically similar (e.g. dog, fog, bog, log). Long-term The storage in sensory memory and short-term memory generally have a strictly limited capacity and duration, which means that information is available for a certain period of time, but is not retained indefinitely. By contrast, long-term memory can store much larger quantities of information for potentially unlimited duration (sometimes a whole lifespan). Whilst short-term memory encodes information acoustically, long-term memory encodes it semantically. Baddeley (1966) found that after 20 minutes, test subjects had the greatest difficulty recalling a collection of words that had similar meanings (e.g. big, large, great, huge). Short-term memory is supported by transient patterns of neuronal communication, dependent on regions of the frontal lobe (especially dorsolateral prefrontal cortex) and the parietal lobe. Long-term memories, on the other hand, are maintained by more stable and permanent changes in neural connections widely spread throughout the brain. The hippocampus is essential to the consolidation of information from short- term to long-term memory, although it does not seem to store information itself. Rather, it may be involved in changing neural connections for a period of three months or more after the initial learning. One of the main functions of sleep is thought to be to improve consolidation of information, as it can be shown that memory depends on getting sufficient sleep between training and test, and that the hippocampus replays activity from the current day while sleeping. For example, if we are given a random seven-digit number, we may remember it for only a few seconds and then forget, which means it was stored into our short-term memory. On the other hand, we can remember telephone numbers for many years through repetition; those long-lasting memories are said to be stored in our long-term memory. Physiology Overall, the mechanisms of memory are not well understood. Brain areas such as the hippocampus, the amygdala, or the mammillary bodies are thought to be involved in specific types of memory. For example, the hippocampus is believed to be involved in spatial learning and declarative learning. Damage to certain areas in patients and animal models and subsequent memory deficits is a primary source of information. However, rather than implicating a specific area, it could be that damage to adjacent areas, or to a pathway traveling through the area is actually responsible for the observed deficit. Further, it is not sufficient to describe memory, and its counterpart, learning, as solely dependent on specific brain regions. Learning and memory are attributed to changes in neuronal synapses, thought to be mediated by long- term potentiation and long-term depression. The coding unit: the neuron Neurons (also known as neurones and nerve cells) are electrically excitable cells in the nervous system that process and transmit information. Neurons are typically composed of a soma, or cell body, a dendritic tree and an axon. The majority of vertebrate neurons receive input on the cell body and dendritic tree, and transmit output via the axon. However, there is great heterogeneity throughout the nervous system and the animal kingdom, in the size, shape and function of neurons. Neurons communicate via chemical and electrical synapses, in a process known as synaptic transmission. The fundamental process that triggers synaptic transmission is the action potential, a propagating electrical signal that is generated by exploiting the electrically excitable membrane of the neuron. The neuron's role as the primary functional unit of the nervous system was first recognized in the early 20th century through the work of the Spanish anatomist Santiago Ramón y Cajal (reviewed in Grant, 2007). Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells. To observe the structure of individual neurons, Cajal used a silver staining method developed by his rival, Camillo Golgi. The Golgi stain is an extremely useful method for neuroanatomical investigations because, for reasons unknown, it stains a very small percentage of cells in a tissue, so one is able to see the complete microstructure of individual neurons without much overlap from other cells in the densely packed brain (López-Muñoz, 2006). Fig1. This is an illustration of Cajal staining of Purkinje cells in the cerebellum. These cells are characterized by a very complex dendritic arborization. Anatomy and histology Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous system, there is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter.